Cyclosporine is a potent immunosuppressive agent, but the widespread usage of this agent is severely hampered by its nephrotoxicity. We have discovered that nephrotoxicity is associated with the appearance of a metabolite(s) that is not present in the blood during normal renal function. The long-term objectives of this study are to determine the role of this and other metabolites of CsA in immunnousppression and nephrotoxicity in an effort to enhance the therapeutic index of the drug. The specific aims of this study are: 1) to identify the metabolites of CsA asssociated with nephrotoxicity in humans; 2) to demonstrate that resistance to nephrotoxicity in mice is due to the absence of this metabolite(s); 3) to purify the metabolites of CsA from human bile: 4) to assess the in vitro immunosuppressive activities of the metabolites; 5) to assess the immunosuppressive and nephrotoxic properties of the metabolites in vivo using mouse allograft and xenograft models; 6) to determine the effects of cyclosporines on human helper and suppressor T cell functions; 7) to determine if CsA directly inhibits human B cell activation, growth, and differentiation in vitro, and 8) to determine if the CsA blocks the expression of early (c-myc, c-fos, IL2, IL2R) and late (c-myb, TFR) activation genes in T cells and B cells. CsA metabolites will be identified in the blood of human recipients during episodes of nephrotoxicity. The metabolites will be purified from human bile by displacement chromatography and given to animals to determine their nephrotoxic and immunosuppressive properties. The purified materials will then be used to study the induction of helper and suppressor cell activity in the allogeneic mixed lymphocyte cultre and will also be tested for their abilities to suppress human B cell activation, growth, and differentiation. Finally, we will use the cyclosporines to study the effects of this drug on gene expression in T and B lymphocytes. This study provide information that will improve the clinical use of CsA and may ultimately lead to the development of less toxic forms of the drug.